The invention relates generally to the placing of conductors on a substrate (e.g., circuit board writing) and, in particular, to placing very fine metallic spheres on a substrate using capillary stream break-up.
Methods of circuit board printing include traditional etching, chemical vapor deposition, focused ion beam writing, micropen direct writing, and drop-on-demand dispensing. Additionally, U.S. Pat. 5,938,102 to Muntz et al. discloses a method of generating molten metal droplets from the phenomenon of capillary stream break-up, charging the droplets, and deflecting the droplets to predetermined locations on a substrate. The method of using droplets generated from capillary stream break-up is several orders of magnitude faster than the other direct write technologies; however, the method is limited by the size of the droplets that are produced.
The generation of droplets from capillary stream break-up has been studied at least as early as Lord Rayleigh in the 1800s. Using the process of capillary stream break-up, the droplets can be produced at very high ratesxe2x80x94typically tens of thousands of droplets per second. Further, the nature of droplet formation due to capillary stream break-up results in highly uniform droplets.
However, conventional methods of formation of metal droplets by capillary stream break-up is limited to the formation of droplets having diameters in excess of 50 microns. A significant limitation on the size of metal balls produced from capillary stream break-up is the size of the orifice from which the capillary stream emerges. Typically, droplets generated from capillary stream break-up have diameters that are roughly twice as large as the diameter of the capillary stream orifice. The production of smaller balls, therefore, typically requires smaller orifices. As the orifice becomes very small, it tends to be more easily clogged by, e.g., impurities in the molten metal. Further, obtaining smaller orifices that are also uniform tends to be difficult and expensive. Current state-of-the-art provides a lower limit of orifice diameter available off-the-shelf and suitable for use with molten metals of 25 microns.
Accordingly, the present invention enables the placing of very fine metallic spheres on a substrate, e.g., direct circuit board writing, wherein the formation of the metallic micro-spheres is due to capillary stream break-up. The present invention further enables the formation of metallic micro-spheres that are significantly smaller than metallic micro-spheres made by conventional methods and, more particularly, to metallic micro-spheres that are significantly smaller than the capillary stream orifice from which the capillary stream emerges, thereby overcoming many of the difficulties that plagued conventional methods.
In accordance with one innovative aspect of the invention, a method of depositing metal onto a substrate comprises directing a capillary stream of molten metal from an orifice by applying an excitation disturbance, wherein the excitation disturbance is determined so that parent droplets and satellite droplets form from the stream due to capillary stream break-up; and directing at least some of the satellite droplets to predetermined locations on the substrate. The satellite droplets can be precisely directed to locations on the substrate by selectively imparting to them an electrostatic charge and passing the satellite droplets through an electric field, which deflects the droplets in predictable amounts. To facilitate the writing of the satellite droplets onto the substrate, the substrate is preferably translatable in a plane substantially orthogonal to the capillary stream (e.g., by being attached to an x-y table).
In accordance with another innovative aspect of the present invention, an electrical assembly comprises a substrate and a plurality of metal balls attached to the substrate, wherein each of the balls preferably has a diameter in a range of about 1.0 to 100 microns, and preferably less than about 25 microns. In accordance with yet another innovative aspect of the present invention, an electrical assembly comprises a substrate and a conductive trace disposed on the substrate, the conductive trace having a width or pitch in a range of about 5 to 100 microns, and preferably less than about 25 microns. The conductive traces of the present invention preferably comprise a plurality of solidified metallic droplets that have fused together to form an electrical connection therebetween.
Advantageously, the parent droplets and the satellite droplets not used may be caught in a gutter and recycled back into the system.
Thus, in accordance with the present invention, metal conductors can be printed directly onto a substrate. By employing satellite droplets instead of the parent droplets, the method can deposit very small metal balls to form very fine pitch (preferably on the order of about 10 microns) conductive trace and ball grid arrays on a substrate. Because droplets from capillary stream break-up are generated at high rates (typically on the order of tens of thousands of droplets per second), printing onto a substrate according to these methods is accomplished in less time than many other solutions. More particularly, droplets may be formed in accordance with the present invention preferably at a rate in a range of about 1000 to 200,000 droplets per second and preferably at a rate greater than 4000 droplets per second. Accordingly, conductive traces may be formed in accordance with the present invention at a rate preferably in a range of about 0.5 to 20 centimeters per second. Further, the high uniformity of droplets generated from capillary stream break-up lead to highly uniform products fabricated according to these methods (e.g., highly uniform trace widths or pitches), which increases the reliability of such products. Specifically, conductive traces may be formed in accordance with the present invention having a pitch or width tolerance in a range of about 3.5 to 5.0 percent.
Other aspects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.